During well-drilling operations, formations are often encountered which lose physical and dimensional integrity when exposed to aqueous drilling fluids (muds). In most cases, such formations comprise reactive shales. Prior to the advent of oil-based, emulsion and invert emulsion muds, great difficulty was encountered in drilling these types of shales. On the other hand, while hydrocarbon-containing muds make drilling through reactive shales possible, they pose safety and environmental hazards and can interfere with well-logging operations.
A variety of water-based drilling additives has been developed over the years in an attempt to deal with shale drilling problems effectively while avoiding the problems encountered when oil-based muds were used. Vail et al., in U.S. Pat. No. 2,133,759, disclose muds containing alkali metal silicates, but sodium is the sole silicate that they disclose using, and they state that not all sodium silicates are suitable. In U.S. Pat. No. 2,146,693, Vietti et al. disclosed a drilling fluid containing one of several sodium salts, including sodium silicate, the sodium salt content of the mud being in excess of 20% by weight. Sawdon, in U.K. Patent Application 2 089 397 A, disclosed an aqueous wellbore fluid for use as a drilling, well completion, packing, perforating or workover fluid containing at least 20% by weight of potassium carbonate; optionally a water-soluble polymeric viscosifier, such as carboxymethylcellulose, was added to the wellbore fluid. Gray et al., in "Composition and Properties of Oil Well Drilling Fluids", 4th Edition (Gulf Publishing Company 1980), reported the use of potassium salts in drilling fluids to suppress the swelling of bentonite, potassium chloride being preferred. Another standard industry reference that describes the shale stabilizing effect of the potassium ion in polymer muds may be found in SPE 10100 "Fundamentals and Use of Potassium/Polymer Drilling Fluids to Minimize Drilling and Completion Problems Associated with Hydratable Clays," by R. P. Steiger, presented at the 56th Annual Fall Technical Conference, Dallas, Tex., Oct. 5-7, 1981. Anderson et al., in U.S. Pat. No. 4,142,595, disclosed shale-stabilizing drilling fluids containing flaxseed gum together with potassium and/or ammonium salts. U.S. Pat. No. 4,447,341, to Block, relates to a drilling fluid containing AlO(OH) and a crosslinked polyvinyl alcohol together with a potassium salt, such as potassium chloride, as a clay-stabilizing composition. Corley et al., in the Oil & Gas Journal, Jan. 27, 1987, described shale inhibition using an aqueous system, maintained at a pH of 9.0 or less, which contains an unidentified nonionic polymer combined with potassium phosphate complex of undefined composition. Lu et al., in U.S. Pat. No. 4,743,384, disclose an aqueous drilling mud consisting essentially of carboxymethyl guar. Lu et al. additionally stated that the thermal stability of their drilling muds can be enhanced up to at least 275.degree. F. upon the addition of a water-soluble metal carbonate such as potassium or sodium carbonate or bicarbonate. However all examples of such compositions were limited to the use of sodium carbonate or bicarbonate.
The current technology of choice for aqueous-based shale-stabilizing muds involves the use of partially hydrolyzed polyacrylamide (PHPA) combined with potassium chloride. Scheuerman disclosed such a combination in U.S. Pat. No. 3,738,437 as part of a regimen for dealing with shale-drilling problems. Systems involving PHPA in combination with various electrolytes, including potassium chloride, at pHs of 8.5 to 10.0, were also reviewed by Clark et al. in SPE/IADC 16081, presented at the Mar. 15-18, 1987 SPE/IADC Drilling Conference in New Orleans, La.
While the systems developed thus far show improvement over the older water-based mud technology, they are sometimes inadequate when difficult shale formations are encountered. Operators are, therefore, forced to revert to hydrocarbon-containing fluids. It is desirable to provide improved additives for water-based muds that would minimize or completely avoid the need for oil-based systems when drilling problem shale formations.